data: add systemd unit file for running daemon

This adds a systemd unit file for running the Calls daemon, and adds a
key to the xdg autostart file to instruct gnome-session to ignore
starting when systemd is present. The expectation is that systemd
distros will enable the Calls daemon as a user service and not rely on
xdg autostart / gnome-session for running/managing the service. Systemd
is better at managing services than gnome-session.

Note: it's important to keep the xdg-autostart file around, some distros
(pmOS on openrc, others) don't use systemd and will still need to rely
on the xdg autostart mechanism for starting the Calls daemon.

Co-authored-by: Guido Günther <agx@sigxcpu.org>

data/calls-daemon.service.in

@@ -0,0 +1,11 @@
+[Unit]
+Description=A phone dialer and call handler (daemon mode)
+PartOf=graphical-session.target
+
+[Service]
+BusName=org.gnome.Calls
+ExecStart=@bindir@/gnome-calls --daemon
+Type=dbus
+
+[Install]
+WantedBy=graphical-session.target